1. Field of the Invention
The present invention relates to a fuel injection system for internal combustion engines, and more particularly the invention relates to a fuel injection system designed for use with an internal combustion engine of a type in which a stratified charge of rich and lean air-fuel mixtures is supplied into the combustion chamber of each cylinder, whereby when the engine is operating with decreased air flow, fuel is sequentially injected into the respective cylinders in accordance with their firing order, whereas when the engine is operating with increased air flow, the fuel is simultaneously injected into all the cylinders.
2. Description of the Prior Art
With the conventional internal combustion engines, it has been customary to maintain the ratio of air-fuel mixture supplied into the combustion chamber of each cylinder lower than that of the ideal air-fuel mixture for complete combustion so as to ensure a positive ignition of the mixture. However, such rich mixtures are subjected to incomplete combustion thus producing various harmful exhaust gases such as carbon monoxides (CO) and hydrocarbons (HC) which have recently been posing a serious social problem. Further, the feeding of such relatively rich mixtures results in an increased fuel consumption. Particularly, at lower engine speeds such as during the starting period of the engine, these problems become even more serious since a very rich mixture must be delivered to the cylinders.
On the other hand, it has been found that these problems of noxious exhaust gases, increased fuel consumption, etc. can be overcome by feeding a rich mixture only around the spark plugs of an internal combustion engine and operating the engine with a lean mixture on the whole. A disadvantage of this method arises when an electronically controlled fuel injection system which is superior both in response characteristic and controllability is used as the fuel feed means. While a simultaneous injection type fuel injection system has the advantage of a simple construction, it is difficult to supply a stratified charge of rich and lean air-fuel mixtures into the combustion chambers of each cylinder. On the other hand, to use a sequential injection type fuel injection system even at higher engine speeds at which the problems of noxious exhaust gases and increased fuel consumption are not so serious, each of the cylinders must be provided with a main computing unit which determines the timing and duration of the opening of the fuel injection valves.